Cellulose (2016) 23:2249–2272
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and Salminen 2009). These chemicals might interfere the fiber water gel and this results in less contact points between the fibrils. Another research group developed vinylformamide copolymers to increase the IWWS (Esser et al. 2008; Gels et al. 2012). The presented results are from paper machines and it is proposed that less breaks in the first open draw are a reference for better IWWS. Various cationic polyacylamides are used to improve dry strength of paper. Despite good results in dry paper, a decrease in IWWS is observed (Alince et al. 2006). The explanation for these phenomena is the steric and electrostatic repulsion of fully poly- acrylamide coated fibers. This results in increased distances between fibers with less friction force in the wet web. If the dosage is ‘‘below full coverage, fiber flocculation occurs resulting in weak spots in the wet sheet’’ (Alince et al. 2006). A selective addition of different additives is a good option for improving IWWS. Salminen added a cationic polyacrylamide (C-PAM) to the short fiber fraction and a cationic starch to the long fiber before mixing them. He observed good results in dewatering and IWWS. The positive effects are explained by the prevented flocculation of the long fibers due to adding the C-PAM to the short fibers including better retention of fines and better sheet formation. In addition it is supposed that ‘‘selective addition of chemicals generated pulp with both cationic and anionic surfaces, thus leading to a greater quantity of molecular level interactions’’ (Salminen 2010). Sev- eral other authors got also good results with selective addition of anionic and cationic polyacrylamides (Sutman 2011), microfibrillated cellulose together with anionic and cationic polyethyleneimine (Szeif- fova and Alince 2003), as well as chitosan, cationic starch and polyvinyl alcohol (Hamzeh et al. 2013). Furthermore the spraying technique in the wet end of the paper machine is rediscovered in several recent publications (Oksanen et al. 2011, 2012; Retulainen and Salminen 2009; Salminen 2010). This technology has the advantage that there are almost no interactions between water contaminants and the used additives as well as a very good chemical retention on the web. The dryness of the web is usually [ 45 % and good results were achieved with CMC (Salminen 2010), Chitosan (Salminen 2010), Guar (Oksanen et al. 2011),
Xyloglucan (Oksanen et al. 2011, 2012) and polyvinyl alcohol (Retulainen and Salminen 2009; Salminen 2010). One disadvantage might be the contamination of the felts in the press section with residues of these additives. In general, while chemical additives can improve the IWWS, their effects are minor compared to those of fiber morphology and dryness. Additionally, most of these additives are hydrophilic (Pelton 2004), with the result that the achievable dryness under given process conditions will be lower, thus limiting the IWWS.
Sheet forming
Studies performed on paper machine forming sections usually emphasize the dewatering speed, retention and web formation. In addition to studies that describe the influence of beating on dewatering and strength (Berger and Schramm 2011; da Silva et al. 1982; Kibblewhite 1973; Lindqvist 2013; Lindqvist et al. 2011, 2012; Lindstro¨m and Kolman 1982; Pikulik 1997), there is an extensive body of research papers on dewatering chemicals and the use of mineral fillers (Alince et al. 2006; de Oliveira et al. 2009; Esser et al. 2008; Ga¨rdlund et al. 2003; Hua et al. 2011; Lindqvist et al. 2009; Pikulik 2000; Sutman 2011; Szeiffova and Alince 2003).
Fig. 10 Sheet structure of unbeaten softwood pulp at SR 12 and 20 % dryness
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